Recently, food-wrapping methods have significantly changed in accordance with changes in food distribution systems and dietary habits, and has created increasingly rigorous demands with respect to wrapping film characteristics.
In the prior art, a biaxially oriented polyamide film has been widely used as a wrapping material for liquid food, liquid-immersed food, frozen food, retort food, paste food, animal and marine products and the like, such as soups, pastes made from konjak flour (konjak), hamburger steaks, fermented soybean pastes (miso), ham, etc. due to its characteristics, such as toughness, excellent gas barrier properties, pin-hole resistance, transparency, easy printability and the like.
In practice, a biaxially oriented polyamide laminate film is prepared by forming a printed layer and an adhesive layer on top of a biaxially oriented polyamide film, and forming thereon a sealant layer by a dry laminating method or an extrusion laminating method. The biaxially oriented polyamide laminate film thus obtained is used to produce a bag, and the opening is heat sealed after the contents are inserted into the bag. Seasonings, such as fermented soybean pastes (miso), soy sauce and the like; moisture-containing food, such as soups, retort food and the like; pharmaceutical products and the like are packaged into such a biaxially oriented polyamide laminate film bag and supplied to consumers.
As the method for producing a biaxially oriented film by the flat method, successive biaxial stretching method and simultaneous biaxial stretching method are known. These methods are also used for producing polyamide film.
However, it is known that the above-mentioned biaxial stretching methods often produce differences in physical properties in the width direction of the film, which is perpendicular to the film running direction. Namely, during the transverse stretching step in the successive biaxial stretching method, or the longitudinal/transverse simultaneous stretching step in the simultaneous biaxial stretching method, longitudinal shrinkage occurs in the tenter due to longitudinal thermal shrinkage stress caused by heating during the heat setting step immediately after transverse stretching, based on the stretching stress in the longitudinal direction caused by the transverse stretching. Since the film edges are gripped by clips and either restrained or allowed to progress in the film running direction, the forming speed of the central part of the film is slower than those of the ends of the film due to the above-mentioned longitudinal shrinkage (bowing phenomenon). Therefore, for example, the difference between the boiling water shrinkage proportions in oblique directions increases at the edge parts of the film, causing a twisting phenomenon when a bag prepared in this manner is subjected to heating or a like treatment, and leads to serious problems.
For solving this problem, it is effective to lower the heat setting temperature to thereby reduce the above-mentioned longitudinal stress. However, lowering the heat setting temperature reduces the adhesion strength between the film and the sealant layer, and is not preferable.